A relay device capable of preventing transmission of an improper message to one or more networks even if improper falsification is made to a program for executing relay processing. If one CAN controller receives a message, a gateway stores the received message in the message storage unit and also sends the message to the processing unit. The processing unit that received the message performs processing necessary for relay of the message by executing a relay program, and sends the message to be relayed to the CAN controller. The gateway compares the message before being sent from the CAN controller to the processing unit with the message sent from the processing unit to the CAN controller, to determine the properness of the message.